“We’ve known for some time that exercise has a role to play in treating symptoms of depression, but this is the first time we have been able to quantify the preventative potential of physical activity in terms of reducing future levels of depression,” said Samuel Harvey, lead author of the study and an associate professor at the Black Dog Institute and University of New South Wales.

The researchers from the Black Dog Institute studied more than 33,000 Norwegian adults for more than 11 years. They used the data from the Hunt Study of Nord-Trøndelag County, which was administered from Jan. 1984 to June 1997, making it one of the broadest population-based health surveys ever conducted.

The “healthy” group of subjects reported how frequent and how intense they participated in their physical activities: without becoming breathless or sweating, becoming breathless and sweating, or exhausting themselves. For a follow-up they were asked to complete a self-report questionnaire, which was the Hospital Anxiety and Depression Scale, to identify any possible symptoms of anxiety or depression. Other influencing factors, such as socio-economic and demographic factors, substance use, body mass index, new onset physical illness, and perceived social support, that may affect the relationship of exercise and mental illness were also considered.

The findings, published in the American Journal of Psychiatry, show that 12 percent of cases of depression could have been prevented if participants exercised for one hour every week.

“Most of the mental health benefits of exercise are realized within the first hour undertaken each week,” said Harvey.

Those who did not exercise at all had a 44 percent more chance of developing depression compared to those who exercised for one to two hours per week. On the other hand, exercise had no protective effect against anxiety. However, the researchers are still uncertain as to why does exercise has this effect on combating depression.

The study came after the Black Dog Institute Exercise Your Mood program of the last September, which encouraged people to enhance their mental and physical health by physical activity.

According to the World Health Organization (WHO), depression is a common, but serious condition that affects over 300 million people of all ages worldwide. It negatively affects how a person feels, thinks, and manages everyday activities, which can lead to suicide in worst case scenarios. Suicide is the second major cause of death in 15- to 29-year-olds with about 800,000 deaths every year. Only about 10 percent of them receive treatments because of lack of resources, lack of trained health-care providers, social stigma linked with mental illnesses, and inaccurate assessment, in which some are too often misdiagnosed and prescribed antidepressants.